Only allow forced fragmentainer breaks at class A break points.

third_party/WebKit/Source/core/layout/PaginationTest.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/layout/LayoutBlockFlow.h"
 #include "core/layout/LayoutMultiColumnFlowThread.h"
 
 #include "core/layout/LayoutTestHelper.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 127 matching lines @@
 {
     setBodyInnerHTML(
         "<div style='overflow:-webkit-paged-y; height:200px; line-height:150px;'>"
         "    <div id='block1'>line1</div>"
         "    <div id='block2' style='padding-top:2px;'>"
         "        <div id='innerBlock' style='padding-top:2px; break-before:page;'>line2</div>"
         "    </div>"
         "</div>");
     EXPECT_EQ(0, strutForBox("block1"));
     EXPECT_EQ(0, strutForLine("block1", 0));
-    EXPECT_EQ(0, strutForBox("block2"));
-    EXPECT_EQ(48, strutForBox("innerBlock"));
+    // There's no class A break point before #innerBlock (they only exist *between* siblings), so
+    // the break is propagated and applied before #block2.
+    EXPECT_EQ(50, strutForBox("block2"));
+    EXPECT_EQ(0, strutForBox("innerBlock"));
     EXPECT_EQ(0, strutForLine("innerBlock", 0));
 }
 
 } // namespace blink